This invention relates to a circuit for controlling the connection of a loudspeaker to a transistor power amplifier and more particularly to a circuit both for disconnecting the loudspeaker from its amplifier to protect it from the excessive d.c. output voltages which may occur during amplifier malfunction, and for disconnecting the speaker during power supply buildup and decay to prevent the transient audio signals generated during these times from being reproduced by the speaker.
In the normal operation of a push-pull transistor power amplifier, the output terminal of the amplifier operates at a zero d.c. potential. This enables the loudspeaker to be directly d.c. coupled to the output circuit of the amplifier. Because of this direct d.c. connection, the speaker may become damaged if an output transistor of the amplifier breaks down or burns out and a high level d.c. voltage is delivered to the amplifier output terminal. The high d.c. voltage when delivered to the speaker may cause speaker damage through the abnormal distension of the driven element or driver coil burn-out.
The control circuit described hereinafter prevents damage to the speaker during such an amplifier malfunction by immediately disconnecting the speaker from the output terminal upon the occurrence of a high d.c. voltage of either polarity at the output terminal of the amplifier. This is accomplished by a transistor switching circuit which under normal amplifier operating conditions maintains normally-open relay contacts between the amplifier and the speaker closed. However, when a d.c. voltage of a predetermined magnitude is present at the output terminal of the amplifier, the switching circuit releases the relay contacts sufficiently quickly to prevent damage to the speaker.
Another common problem in amplification systems is one involving undesirable noise caused by the delay of the main power supply for the amplifier in either building up to its operating voltage, or returning to a zero voltage once the power supply is turned off. The long time required for the main power supply to reach operating voltage and to decay to zero voltage are primarily due to the time necessary to charge and discharge the filtering capacitors in the rectifier circuit of the main power supply.
As the main power supply voltage builds up, the amplifier circuits respond to the increasing power supply voltages by generating distorted audio output signals. These distorted audio signals are reproduced by the speaker if the speaker is connected to the amplifier during the power supply warm up period. Likewise, when the main power supply is turned off, decaying audio signals are generated by the amplifier as the power supply voltage decays. Additionally, switching transients occurring when the main power supply is switched on can be heard as a loud thump.
The distorted audio signals and/or power supply noise created during main power supply warm-up or shut down are prevented from reaching the speaker by disconnecting the speaker during these times. In the case of the warm-up noise, the closing of the normally open contacts between the loudspeaker and the amplifier is delayed by an RC timing circuit for a time exceeding the expected warm-up time for the main power supply. With respect to the noise generated when the main power supply for the amplifier is shunt off, the relay contacts between the speaker and the amplifier are quickly opened upon disconnecting the main power supply from the a.c. line. This prevents distorted signals from being reproduced by the speaker. For this purpose, the control circuit derives its only source of power from an auxiliary a.c. power supply having a substantially zero buildup and decay time, so that when the main power supply switch is opened power to the control circuit is immediately removed, thus deactivating the relay. The buildup and decay time constants of the auxiliary power supply are short compared to those of the main power supply so that the speaker is disconnected from the amplifier output terminal prior to any substantial decay of the output voltage from the main power supply. Thus, the distorted signals from the amplifier during the decay of the main power supply are prevented from reaching the loudspeaker.